The development of mouse hybridoma technology has allowed the production of antibodies (Ab) specific for a wide range of antigens. Mouse monoclonal antibodies (mAb) have been used extensively for diagnosis and, in a few cases, for human therapy and in vivo diagnostics. Administration of murine antibodies to humans has been observed to induce a strong human anti-mouse antibody response (HAMA) after single or repeated treatments, thus precluding long-term treatment using these antibodies. Moreover, rodent antibodies are rapidly cleared from human serum and often do not interact effectively with the human immune system. Since human hybridomas are generally unstable and secrete low amounts of antibodies (frequently IgMs), considerable effort has been directed at rendering foreign antibodies (e.g., murine antibodies) more similar to those of the host to which they are administered (e.g., a human). Alternatives to human hybridoma-derived antibodies have been developed in which mouse immunoglobulin sequences (e.g., constant regions) are replaced with corresponding sequences derived from human immunoglobulin genes. Two examples of this type of antibody are (1) chimeric mAbs, in which murine variable regions are combined with human constant regions (Morrison et al., Proc. Natl. Acad. Sci. USA 81:6851-6855, 1984; Boulianne et al., Nature 312:643-30 646, 1984), and (2) humanized mAbs, in which murine CDRs (complementarity determining regions) replace the corresponding sequences in human ixmunoglobulins (Jones et al., Nature 321:522-525, 1986; Co et al., Nature 351:501-502, 1991). These engineered antibodies retain their target specificity and generally exhibit reduced HAMA responses when injected into patients. In addition, desired effector functions of antibodies for certain clinical applications can be obtained by using constant regions corresponding to the appropriate immunoglobulin isotype.
Despite these advances, cloning of variable region sequences has been a limiting step in the rapid construction of chimeric and isotype switched antibodies. Polymerase chain reaction (PCR) amplification of immunoglobulin heavy and light chain variable regions has facilitated this step. However, the high degree of DNA sequence polymorphism in leader and variable sequences of both heavy and light chain genes has required the preparation of complex sets of degenerate primers (Jones et al., Bio/Technology 9:88-89, 1991; Kettleborough et al., Eur. J. Immunol. 23:206-211, 1993; Le Boeuf et al., Gene 82:371-377, 1989; Orlandi et al., Proc. Natl. Acad. Sci. USA 86:3833-3837, 1989). In the case of 5' primers, these primers have usually been designed to correspond to the first framework of the variable region (FR1) and, in a few cases, to the leader peptide sequence (L). The 3' primers have usually been designed to correspond to the framework 4 (FR4) region, which displays limited polymorphism, or to the constant region, in which conserved, isotype-specific sequences are easily identified. Although complex sets of 5' and 3' primers have been designed, they do not always match the DNA template completely (Gavilondo-Cowley et al., Hybridoma 9:407-417, 1990; Leung et al., BioTechniques 15:286-292, 1993). Native sequences of the immunoglobulin heavy and light chain genes may therefore be altered in the FR1 and/or FR4 regions by the PCR amplification process. Modifications of the N-terminal region of an immunoglobulin, particularly the light chain variable region (VL), in which the amino acid at position two is part of the predicted canonical structure for CDR1 (Chothia et al., Nature 342:877-883, 1989), have been shown to drastically reduce the affinity of immunoglobulins for their antigens. Moreover, expression levels of the recombinant antibodies may also be altered when mutations occur in the leader peptide. In most studies involving PCR amplification of immunoglobulin H (heavy) and .kappa./.lambda. (light) chain variable regions using these primers, cDNA templates were used, resulting in the generation of fragments containing incomplete VH and VL sequences, which may or may not be linked to part of the constant region.